Fluted surface heat exchanger

ABSTRACT

A heat exchanger suitable for use as a recuperator for radiant tube burners includes a fluted heat exchange surface. The fluted surface is made from flat sheet stock which is repeatedly folded along parallel lines from edge to edge to form a series of longitudinally extending fluteways. Individual fluteways may be closed at both ends, open at both ends or open at one end and closed at the other end. The fluted surface is formed into a tube and positiond within an outer, jacket tube. The fluted surface tube is connected to a core tube, with the fluteways extending longitudinally through the annular space between the core tube and the jacket tube. Indirect heat exchange between a hot gas stream and a cold gas stream is effected through the fluted surface by flowing one of the gas streams on one side of the fluted surface and the other on the opposite side of the fluted surface. The flow of the two gases may be either concurrent or countercurrent. Core flow baffles may be utilized to confine the flow of gases substantially within the fluteways to enhance heat exchange.

United States Patent [19] Kerr [451 Nov. 18, 1975 1 1 FLUTED SURFACE HEAT EXCHANGER [75] Inventor: Elmer L. Kerr, Butler Twp.,

Columbiana County, Ohio [73] Assignee: The Electric Furnace Company,

Salem, Ohio 22 Filed: June 20, 1974 21 Appl. No.: 481,356

[52] U.S. Cl 432/223; 126/91 R; 165/179; 432/175 [51] Int. Cl. F24J 3/00 [58] Field of Search 432/175, 179, 223; 126/91 R; 165/179 [56] References Cited UNITED STATES PATENTS 1,128,146 2/1915 Jacobs 432/179 1,775,173 9/1930 Phelps et a1... 126/91 R 2,188,133 l/l940 Hepburn 165/179 X 2,374,609 4/1945 McCollum.... 165/179 2,797,554 7/1957 Donovan 165/179 X 3,177,936 4/1965 Walter 165/179 3,200,848 8/1965 Takagi 165/179 X 3,473,348 10/1969 Bottum 165/179 X 3,828,761 8/1974 Rich 126/91 R Primary Examiner-John J. Camby Attorney, Agent, or FirmMeyer, Tilberry & Body [57] ABSTRACT A heat exchanger suitable for use as a recuperator for radiant tube burnersincludes a fluted heat exchange surface. The fluted surface is made from flat sheet stock which is repeatedly folded along parallel lines from edge to edge to form a series of longitudinally extending fluteways. Individual fluteways may be closed at both ends, open at both ends or open at one end and closed at the other end. The fluted surface is formed into a tube and positiond within an outer, jacket tube. The fluted surface tube is connected to a core tube, with the fluteways extending longitudinally through the annular space between the core tube and the jacket tube.

Indirect heat exchange between a hot gas stream and a cold gas stream is effected through the fluted surface by flowing one of the gas streams on one side of the fluted surface and the other on the opposite side of the fluted surface. The flow of the two gases may be either concurrent or countercurrent.

Core flow baffles may be utilized to confine the flow of gases substantially within the fluteways to enhance heat exchange.

38 Claims, 24 Drawing Figures Nov. 18, 1975 Sheetlofll 3,920,383

U.S. Patent U.S..Patent Nov. 18, 1975 Sheet20fll 3,920,383

um mm mm m wmm Patent Nov. 18, 1975 Sheet 3 of 11 3,920,383

(th /70 [68 w 1 A FIG 4A U.S. Patent Nov. 18, 1975 Sheet40fl1 3,920,383

UAUUnUnUUn U 0000 US. Patent Nov. 18, 1975 Sheet 5 of 11 Sheet 6 of 11 U.S.- Patent Nov. 18, 1975 Patent Nov. 18,1975 Sheet70f11 3,920,383

FIG. l0

FIG. II

U.S..Patent Nov. 18, 1975 Sheet80f11 3,920,383

Sheet 9 of 11 U.S. Patent Nov. 18, 1975 Nov. 18, 1975 Sheet 10 of 11 US. Patent islimited. Another shortcomingis'theinherent inefficiency of FLUTED SURFACE HEAT EXCHANGER The present invention relates to a novel, fluted surface heat exchanger which is particularly useful as a heat exchanger for radiant tube heaters of the type sometimes used in annealing and other heat treating furnaces. More particularly, the invention is concerned with a heat exchanger which is particularly adapted to be utilized in radiant tube burners to recoup some of the heat otherwise lost with the hot flue gases. Although the invention has other, broader applications to indirect heat exchange in general, it will accordingly be described with particular reference to such recuperative heat exchangers, sometimes called recuperators. The recuperators are generally used to preheat combustion air utilized in radiant tube heaters by indirect heat exchange with the hot flue gases prior to discharging the same.

The utilization of combustion air preheating by indirect heat exchange with flue gases is of course well known in the art. Such heat recovery is important economically and becomes highly important in the context of conserving scarce and dwindling fuel supplies by maximizing heat energy recovery. The usual radiant tube heater has an entry leg which enters the furnace enclosure, an intermediate radiatingsection within the enclosure, and an exit leg which exits therefrom. 'Thus,

the radiant tube heater has a generally U-shaped configuration, although the intermediate radiating section may be serpentine in order to increasethe radiant surface area within the furnace enclosure. Combustion air and a suitable fuel, usually gas or oil, are introduced into the inlet leg and ignited therein by a pilot light or other ignition mechanism. The combustion of the fuel within the tube heats the tube so that it provides radiant heat to the furnace enclosure. The hot gases of combustion exit from the exit leg of the radiant tube heater to a suitable vent discharge.

In accordance with the prior art, preheating of inlet combustionair for radiant tube heaters is attained by jacketing a cold air inlet pipe with a heat exchanger .sleeve in the radiant tube. The cold inlet air is heated by indirect heat exchange with'exiting hot flue gases.

Generally, see US. Pat. No. 3,079,910 (Bloom et a1) and 3,187,740 (Jones et al); Fed. Rep. "of Germany Pat. Nos. 1,074,082, 1,066,312 and 1,111,763; and a German magazine article entitled Die Entwicklung und die .Bedeutung des Strahlheizrohes in 'modernen Industrifonscribing jet impingement recuperators, which also con- I 2 ids. Another expedient is to provide a tube with solid wall fins extending therefrom. Such extended tube construction, while efficient, is costly. Further, such construction is usually not practical within the limited space available in a radiant tube heater. Further, as described hereinbelow, solid-wall finned tubes as well as smooth wall cylindrical recuperators are less efiicient heat exchangers than those provided by the invention.

Another prior art expedient is the use of jetted air impingement heat exchangers, but these are bulky, require installation of equipment outside the tube and generally require higher pressure air. An article detains a good discussion of the utilization of recuperators in general, is A Jet Impingement Recuperator for Gas Fired Radiant Tube Furnaces by S. E. Nydick'and L. -J Lazaridis, published by The American Society of Mechanical Engineers, (ASME), 345 East 47th Street, New York, NY. 10017. (This article, the disclosure of which is incorporated by reference herein, was presented at the Winter Annual Meeting of ASME at Detroit, Mich., Nov. 11-15, 1973.)

It is accordingly an object of the present invention to .provide a novel and improved structure for heat exbaa by lng.,Franz l-lelfurt, pages 336-340 of the October 1956 issue of 'Organdes Gas Warme Instituts Essen published by Vulkan-Verlag 'Dr. W. Classen, Essen, Fed. Rep, of Germany.

The prior art suffers from a number of shortcomings.

One shortcoming is that due to exigencies of furnace design; the total length of the heat exchanger sleeve and, consequently, the availableheat exchange surface,

heat transfer between two gaseous streams flowed within smooth wall tubesof limited heat exchange surface, wherein a central core of'the gas is insufficiently heated (or cooled) with the majority of heat exchange occurring in the laminar layers adjacent the heat exchange surface. s

: Of course, the concept of extended surface heat exchangers: is well known. Extension of the surface is usually attained by employing a number of small diameter tubes instead of a single larger tube for one of the fluchangers particularly adapted for use as heat recuperators for radiant tube heaters. It is a further object of the invention to provide a simple, novel and highly efficient heat exchanger design which increases the efficiency of heat transfer between fluid streams. It is another object to provide such a heat exchanger which is compact, readily contained within a leg of a radiant tube heater and which utilizes laminar flow of the fluid streams, ob-

viating the need for jetting and higher pressures. Other objects and advantages of the invention will become apparent from the following description thereof.

In accordance with the present invention, there is provided an extended surface heat exchanger comprising a fluted sheet formed into a conduit around a core tube, with the fluteways extending generally longitudinally of the core tube, and a jacket tube enclosing the fluted sheet conduit. Two fluids are passed in indirect heat exchange, one on one side of the fluted sheet and the other on the opposite side of the fluted surface.

In accordance with one aspect of the invention, the fluted sheet is made from flat sheet stock folded and bent along selected fold lines to form a fluted sheet having a series of parallel individual fluteways thereon, in the manner of corrugated sheet. The fluted sheet may be formed into a conduit of any desired cross-sectional configuration, e.g., circular, oval, a polygon, etc. (By polygon is meant a closed plane figure bounded by straight lines. The term polygon includes a rectangle and the term rectangular includes the special case of a square.) The fluteways may be formed to be straight or spirally, helically or otherwise disposed about the conduit formed from the fluted sheet. Preferably, the fluteways are parallel to eachother, substantially straight and disposed parallel to the longitudinal axis of the conduit formed from the fluted sheet. The jacket tube and the core tube may also be of any desired cross-sectional configuration, e.g., circular, oval, polygonal, etc. Preferably, but not necessarily, the cross-sectional configurations of the jacket tube, core tube and fluted means to the fluteway ends.

In one aspect of the invention each fluteway is open at one end and closed at its other end, with open ends alternating with closed ends at both edges. Altematively, fluteways open at both ends alternate with fluteways closed at both ends.

In another aspect of the invention, the core tube has one or more openings formed therein to admit gas therethrough. The opening may take any suitable shape, e. g., perforations in the tube wall or an open section between segments of the tube.

In accordance with yet another aspect of the invention, core baffles are utilized to confine the flow of the fluids generally within the fluteways of the fluted surface. In another aspect, the invention provides for laminar flow of the fluids therethrough.

Generally, a fluted sheet tube is provided which is contained within a jacket tube to define a first fluid flow space between the outside fluteways and the interior of the jacket tube. A core tube is provided disposed within the fluted sheet tube to define a second fluid flow space between the inside fluteways and the outside surface of the core tube. Outlet conduits conduct the fluids from the heat exchanger.

The invention may be better understood with reference to the following detailed description of preferred embodiments thereof, which are intended to exemplify the invention and not to limit the same, and to the attached drawings wherein:

FIG. 1 is a plan view with parts broken away of a radiant tube heater schematically showing a recuperator in accordance with the invention included therein;

FIG. 1A is a section view taken along line AA of FIG. 1;

FIG. 2 is a partial plan view with parts broken away showing details of a countercurrent flow recuperator in accordance with the invention and usable in the heater of FIG. 1;

FIG. 2A is a section view taken along line AA of FIG. 2;

FIG. 2B is a partial view showing an alternate form of construction of a portion of the recuperator of FIG. 2;

FIG. 3 is a plan view of sheet stock showing the fold lines along which the sheet is folded to form the flutes therein, and showing slit lines along which the stock is slit to close selected flute ends;

FIG. 4 is a perspective view of a portion of the fluted sheet stock of FIG. 3 after folding and slitting thereof;

FIG. 4A is an end view in elevation taken along line AA of FIG. 4;

FIG. 5 is a perspective view of a portion'of the sheet stock of FIG. 4 after the closing of selected flute ends thereof;

FIG. 6 is a plan view of perforated sheet stock prior to the rolling thereof into a tube to form a perforated core tube;

FIG. 7 is a perspective view with parts broken away of the hot gas entry end of the recuperator of FIG. 2;

FIG. 8 is a perspective view with parts broken away of the cold air entry end of the recuperator of FIG. 2;

FIG. 9 is a view generally corresponding to that of FIG. 2 but showing a second, concurrent flow embodiment of the invention, also usable in the heater of FIG.

FIG. 9A is a sectional view taken along line AA of FIG. 9;

FIG. 9B is a sectional view taken along line B-B of FIG. 9;

FIG. 10 is a plan view of sheet stock showing the fold lines along which the sheet is folded to form the flutes therein, and slit lines along which the stock is slit to close selected flute ends in a pattern different from that of the sheet stock of FIG. 3;

FIG. 11 is a perspective view of a portion of the sheet stock of FIG. 10 after the forming thereof into a fluted extended surface with selected flute ends thereof closed;

FIG. 12 is a side view in elevation with parts broken away of another embodiment of a heat exchanger in accordance with the invention;

FIG. 12A is a section view taken along line AA of FIG. 12;

FIG. 13 is a section view in elevation of yet another embodiment of a heat exchanger in accordance with the invention;

FIG. 13A is a section view taken along line AA of FIG. 13;

FIG. 13B is a section view taken along line B-B of FIG. 13;

FIG. 14 is a cross section schematic view of a recuperator in accordance with the invention of rectangular cross section; and

FIG. 15 is a schematic cross section view in elevation of a fumace employing a recuperator in accordance with the invention.

Referring now to FIG. 1, a fumace enclosure is generally indicated at A as formed between furnace wall B and furnace wall C. A radiant tube heater D of generally U-shape has an entry leg 10, a return section 12 and an exit leg 14. Entry leg 10 and exit leg 14 pass through furnace wall B via, respectively, openings 11 and 13 and are supported therein in sliding engagement to accomodate thermal expansion and conu'action. Return section 12 is supported by mounting stud 16 which is welded thereto. Stud 16 is mounted in sliding engagement within bore 18 in furnace wall C. Obviously, any suitable support means can be used.

Exit leg 14 is attached, by welding (or other gas tight means) to a mounting plate 36 which is in turn attached to furnace wall B by mounting brackets 38, 38. Entry leg 10 passes through an opening 39 in mounting plate A fuel pipe 20 and a pilot light tube 22 are mounted in the entry end of entry leg 10 by bracket 26 and end plate 28. An expansion bellows 30 is included in entry leg 10, as is well known in the art, to help accommodate thermal expansion and contraction. The inner (furnace) end of expansion bellows 30 is attached to mounting plate 36 and the outer end to the outside of entry leg 10 near inlet section 46 thereof. A peep sight 32 is mounted in end plate 28 for visual inspection of the combustion, and a combustion air distribution plate 24 is mounted near the endpipe 20.

Exit leg 14 has a number of refractory blocks 34 disposed therein to block the central core portion of exit leg 14 against gas flow therethrough, as will be more fully described hereinbelow.

A heat exchanger or recuperator 40 in accordance with the invention is schematically shown positioned at the exit end of exit leg 14.

A cold air inlet 42 is connected in flow communication with heat exchanger 40, as is a heated air crossover pipe 44. Heated air cross-over 44 is connected in flow communication to an inlet section 46 of entry leg 10.

A discharge section 48 of exit leg 14 is connected in flow communication to a gas discharge flue 50.

In operation, a suitable fuel such as oil or gas is admitted through fuel pipe 20 into entry leg 10. Heated combustion air, obtained as described below, is admitted through cross-over pipe 44 into entry leg 10. The fuel and heated combustion air admix, and the admixture is ignited by a pilot light (or other suitable means) at the end of pilot light tube 22. The combustion of the fuel within radiant tube heater D heats it to radiate heat into furnace enclosure A. The combustion products travel in the direction indicated by the arrows 52 through entry leg 10, exit leg 14, recuperator 40 and out through gas discharge flue 50.

Ambient or cold air is admitted through cold air inlet 42 and passed in indirect heat exchange with the hot combustion gases in recuperator 40. The heated air exits therefrom via cross-over pipe 44 into entry leg as described above.

Referring to FIG. 2, a recuperator 40 in accordance with the invention and usable in the radiant tube heater of FIG. 1 is shown in some detail. Exit leg 14, which is broken away to show recuperator 40, extends through opening 13 in furnace wall B as described hereinabove. A core tube 54 has a perforated section 66 at one end thereof, perforated section 66 having a plurality of perforations 80 formed in the wall of tube 54 to constitute an opening therein.

Core tube 54 has a fluted sheet 56, which has been formed into a tube, wrapped therearound, the fluted sheet extending for a substantial portion of the length of core tube 54. The inside tubular periphery of fluted sheet 56 closely circumscribes the exterior wall of core tube 54. The outside tubular periphery of fluted sheet 56 is itself closely circumscribed by the interior wall of exit leg 14 and that of its associated discharge section 48. These interior walls comprise a jacket tube which encircles tubular fluted sheet 56.

As best seen in the cross-sectional view of FIG. 2A, fluted sheet 56 is wrapped around tube 54 and the abutting edges of sheet 56 are joined to each other along their length to form sheet 56 into a tube. The tubular structure of sheet 56 has an inner surface 58 and an outer surface 60. A series of outside fluteways 62 are formed between outer surface 60 of fluted sheet 56 and'the jacket tube formed by the interior walls of exit leg 14 and its discharge section 48. A series of inside fluteways 64 are formed between the inner surface 58 of sheet 56 and the outside wall of core tube 54. The

, fluteways of sheet 56 are defined by peaks 68 and troughs 70 formed therein.

Referring to FIG. 3, flutes sheet 56 is shown while still in flat, unfluted form, but with markings thereon to indicate the necessary folds and cuts to form the required fluteways therein.

Fold lines to form peaks 68 are shown in FIG. 3 as heavy lines indicated by the numeral 68. The fold lines are equally spaced and parallel, and extend from the left hand edge I of sheet 56 (as viewed in the drawing) to the right hand edge r. Fold lines to form troughs 70 alternate with the lines 68 and are shown as light lines indicated by the numeral 70. These also are a series of equally spaced parallel lines extending from edge I to edge r. FIG. 3 shows sheet 56 in plan view looking down on the outside surface 60 thereof. Accordingly, fold lines 68 are the lines along which the peaks 68 (FIG. 2A) will be formed by bending sheet 56 downwardly thereabout. Fold lines 70 are the lines along 6 which troughs 70 will be formed by bending sheet 56 upwardly thereabout. The fold lines are generally parallel to top edge t and bottom edge b of sheet 56.

Dotted slit lines 72 in FIG. 3 indicate that the portions of fold lines 70 therebetween are to be slit. Dotted slit lines 74 similarly define the corresponding portions of fold lines 68 which are to be slit.

Dashed lines 75 and 77 indicate peripheries defined by the deepest extent of, respectively, slits 72 and 74. As more fully explained hereinbelow, conduits connected to the ends of the tube formed by rolling and welding sheet 56 into tubular form must extend to or beyond the periphery lines 75, 77 from the respective ends of the tube, to close off the fluteway ends and prevent cross flow between inside and outside fluteways.

It should be noted that for clarity of illustration, the distance between adjacent fold lines in FIG. 3 (and in FIG. 10) is greatly exaggerated as compared to the length from edge I to edge r. In actual practice, typical dimensions for sheet 56 might be a length of 24 inches from edge I to edge r, and a distance of inch between adjacent fold lines, this distance being indicated by the dimension 0 in FIG. 3. Naturally, any dimentions convenient for the particular heat exchanger size desired in a given case may be employed.

Sheet 56 is folded by simple, known metal fabricating techniques into a corrugated or fluted form as shown in FIG. 4 and FIG. 4A. It will be appreciated that the fold and slit lines shown in the drawing are for illustration purposes only and the actual sheet material normally is not, and need not, be so marked. The folding forms a series of parallel fluteways in fluted sheet 56, with alternating peaks 68 and troughs 70. Along edge I, troughs 70 are slit for a short distance from edge I, say of an inch. Along edge r peaks 70 are similarly slit for of an inch in from edge r. As may best be seen in FIG. 4A, when sheet 56 is rolled into tubular form by bending edges t and b toward each other as shown by the arrows, top edge t and bottom edge b may be joined along their respective lengths to close the tube and to form another peak 68 at their joined respective edges. A short length at edge r may be left unjoined to form a slit therein corresponding to the other slits 74.

Referring now to FIG. 5, fluted sheet 56 is shown after the closing of the flutes therein. As with the other.

figures, the fold lines and slit lines are indicated thereon to assist in understanding the illustration. The sheet material adjacent slit lines 72 in troughs 70 is pinched together in a manner shown to give a scalloped shape to the edge of the sheet. The segments of edge I thus brought into contact with each other are welded together along weld lines 76, the extent of a typical one of which is indicated by a bracket in FIG. 5. Thus, inside fluteways 64 are closed along edge I of sheet 56, and outside fluteways 62 are left open along edge 1.

Along right hand edge r, a similar technique is followed. At this edge however, slit lines 74 have been cut in the edges of peaks 68 instead of in troughs 70, and the material of a given flute wall is pinched and folded in the direction opposite that from which it was folded at edge I. The pinched-together material is welded along weld lines 78, the extent of a typical one of which being shown by a bracket. In this manner, outside flute ways 62 are closed at edge r and inside fluteways 64 are left open at edge r.

When fluted sheet 56 is to be assembled about core tube 54, top edge t and bottom edge b are welded together along their respective lengths, as described 7 above.

It should be noted that the structure of fluted sheet 56, with individual fluteways being open at one end and closed at their other end, is entirely reversible. That is, if the fluted sheet of FIG. is reversed to exchange the positions of the left hand edge l with the right hand edge r, and fluted sheet 56 is then formed into a tube by bringing edge t together with bottom edge b in the direction opposite to that shown by the arrows in FIG. 5, the identical structure is obtained.

Core tube 54 may also be formed from flat sheet stock. Referring now to FIG. 6, core tube 54 is shown in flat sheet from prior to being formed into a tube. A sheet of suitable stock material has a series of perforations 80 punched therein. The perforations are positioned in a regular pattern along a selected portion of the sheet stock to constitute a perforated section 66 therein. As viewed in FIG. 6, the sheet stock is seen to have a top edge I and a bottom edge b. After holes 80 are punched therein, the sheet stock is rolled into a tube and edge t is joined to edge b by welding or the like. This provides a convenient and efficient manner of manufacturing a tube perforations at selected areas thereof. Naturally, in other embodiments, it may be desired to have an opening in the core tube at both ends of the tube or at any other areas of the tube to meet a required design. Instead of a perforated section, the opening may comprise a slot or an open section formed by segmenting the core tube. For example, FIG. 2B shows an alternate form of construction wherein end wall 55 forms part of a cup-like segment spaced from the remainder of core tube 54 to form an open section 80 therein.

Referring again to FIGS. 2 and 2A, core tube 54, with perforations 80 contained in perforated section 66 at one end thereof, has fluted sheet 56 wrapped therearound in a substantially tubular configuration, as best seen in FIG. 2A.

Edge r of sheet 56 is placed adjacent air inlet 42 and edge I is placed perforated section 66 of tube 54. Accordingly, fluteways 64 are open and fluteways 62 closed at the air inlet 42 end of fluted sheet 56.

The scalloped slits 72 (FIG. 5) at edge I of fluted sheet 56 are continuously welded (or otherwise fastened in a gas-tight manner) to that end of core pipe 54 which contains perforated section 66, as best seen in FIG. 7. It is essential that all perforations 80 be thus sealed beneath the inside surface of fluted sheet 56 to prevent gas flow communication between exit leg 14 and transfer 84.

A series of reinforcing bands 82 are secured about the periphery of fluted sheet 56 to prevent gas flow communication between exit leg 14 and transfer pipe 84.

A series of reinforcing bands 82 are secured about the periphery of fluted sheet 56 by being tack welded to the peaks 68 thereof. Reinforcing bands 82 strengthen the structure and help to hold the flutes in their proper, spaced apart relation. It will be noted, particularly from FIG. 2A, that the folds in sheet 56 are not a sharp crease, but are curved along a substantial radius of curvature to avoid overstressing the material.

The end of core tube 54 is sealed by a circular end plate 55 which is welded to the end of core tube 54 which contains perforated section 66.

Referring to FIG. 2 and 7, a hot air transfer pipe 84 having a flange 86 at one end thereof is coaxially disposed within core tube 54. Flange 86 of transfer pipe 8 84 may be welded in place within core tube 54 with flange 86 being disposed interiorly of perforated section 66. At its opposite end, hot air transfer pipe 84 is connected in flow communication with hot air crossover pipe 44.

Referring to FIG. 8, cold air inlet 42 passes through the wall of exit leg 14 and is connected in flow communication with air entry chamber 88. Air entry chamber 88 comprises a short cylinder closed at one end by a circular end wall 90 and having its open end 91 fitted over (welded to) the outer periphery of band 82. The scalloped slits 74 (FIG. 5) at edge r of fluted sheet 56 are continuously welded (or otherwise fastened in a gas-tight manner) to the interior of reinforcing band 82.

It is essential for proper operation that air entry chamber 88 (and, in the embodiment shown, its associated band 82) extend beyond the periphery defined on tubular fluted sheet 56 by the inside ends of slit lines 74, i.e., that the entire length of slits 74 be sealed. This is necessary in order to seal the annular flow space defined between the interior of air entry chamber 88 and the exterior surface of core tube 54, from the flow of hot gases in outside fluteways 62. This periphery is indicated by dashed line 75. As a matter of practical construction technique, the edge of open end 91 of air entry chamber 88 actually extends somewhat beyond the periphery to a periphery line indicated by the dashed line 75'.

In operation, ambient cold air is admitted via inlet 42 and flows therefrom into air entry chamber 88 as indicated by the double lined arrows 53 (FIGS. 1 and 2). The cold air enters inside fluteways 64 (FIGS. 2A and 5) and travels the length thereof, emerging into core tube 54 through perforations in the perforated section 66 thereof (FIGS. 2 and 7).

Heat transfer from the inlet hot combustion gases to the air being heated is also effected through end plate 55, which helps to reduce the temperature thereof thereby enhancing the serviceable life of the equipment.

It should be noted (FIG. 7) that fluted surface 56 entirely covers all the perforations 80 in perforated section 66 of core tube 54. The inside ends of slits 74 may be considered to define a periphery about core tube 54 illustrated by dashed line 77. Perforated section 66 ends at periphery 77. As a practical construction matter, the perforation 80 closest to the end of tube 54 should not extend beyond a periphery line 77. In this manner, the hot gases flowing through outside fluteways 62, as indicated by the single line arrows 52, are prevented from entering perforations 80 and core tube 54. The cold air flowing'inside fluteways 64 is heated by indirect heat exchange with the countercurrent flow of hot gases through outside fluteways 62, heat transfer occurring through the thin wall of fluted sheet 56. Because of the large surface area provided by the fluted construction, the inlet air is heated to high temperatures by the time it enters core tube 54 through perforations 80. The heated air flows through hot air transfer pipe 84 in a direction opposite to its travel through inside fluteways 64. The annular space between transfer pipe 84 and tube 54 substantially prevents heat exchange between cold air in fluteways 64 and heated air in transfer pipe 84. From transfer pipe 84, the heated air flows into heated air crossover pipe 44.

As shown in FIG. 1, from crossover pipe 44, the heated air enters inlet section 46 of entry leg 10 9 wherein it is admixed with fuel admitted through fuel pipe 20 and combusted in entry leg 10. The gaseous products of combustion flow through radiant ,tube D to exit leg 14 and then in countercurrent heat flow transfer with inlet cold air, as described hereinabove.

The flue gases, cooled by virtue of having given up a portion of their heat to the incoming cold air, exit from outside fluteways 62 around the outside of air entry chamber 88 (thus providing further heat exchange) and into discharge section 48 of exit leg 14 (FIG. 2). From discharge'section 48 the heat exchanged combustion product gases enter a gas discharge flue 50 (FIG.

1) from which the cooled gases may be discharged to the atmosphere, or further treated for pollution control and/or energy recovery purposes or the like.

In order to take fullest advantage of the extended heat exchange surface efficiently and economically I provided by the fluted construction of fluted sheet 56,

it is desirable to have all or most of the air and gas flow occur within the provided fluteways and not, for example, in the interior core portion of tubular sheet 56. It will be noted that the described construction provides this feature. End plate 55 (FIG. 7) of core tube 54 diverts the entirety of the hot gas flow (arrows 52) into the restricted annular space, provided between the jacket tube formed by the interior of exit leg 14 and exterior surface 60 of fluted sheet 56. In order to increase the velocity of the hot gases 52 flowing through exit leg 14, a series of refractory blocks 34 (FIG. 1) are positioned in exit leg 14 ahead of (as sensed moving in direction of the hot gas flow arrows 52) the area of hot gas entry into outside fluteways 62 adjacent end plate 55.

As shown in FIG. 1A, refractory blocks 34 have extended legs 35 formed thereon to center the body portion 37 in the central core portion of the pipe, in this case entry leg 14. This blocks the central core portion of entry leg 14 to gas flow, the gas being compelled to flow around refractory blocks 34 at increased velocity in the roughly triangular-shaped segments defined between legs 35 and the interior wall of entry leg 14. Naturally, any suitable shape of refractory block or other material can serve this corebuster function which increases the gas velocity and therefore the heat transfer in the relatively cooler exit leg of the burner.

Referring now to FIG. 9, there is shown a second embodiment of the invention which embodies concurrent flow of the cold inlet air and the hot combustion product gases. Parts similar or identical to thoseof the embodiment of FIG. 2 are identically numbered. Thus, recuperator 40 is positioned within exit leg 14 at that portion thereof passing through furnace wall B. Core tube 54 is positioned within exit leg 14 and has fluted sheet 56 formed into a tube encircling the periphery thereof, with fluteways 62 and 64 (FIG. 9A) extending longitu dinally thereof.

In this embodiment, core tube 54 has two openings 3 comprising perforations 80 contained in two perforated sections 65 and 66. Perforated section 65 is located adjacent edge I of tubular sheet 56, at the end of recuperator 40 adjacent furnace enclose A. Perforated section 66 is an intermediate section located adjacent edge r of sheet 56, the cold air entry end of recuperator 40. Obviously, the openings could be provided by open sections in a segmented core tube 54 or by any other suitable construction. Core tube 54 has an end plate 55 closing off the furnace end thereof. (FIGS. 9 and 9B).

As is clearly apparent from FIG. 9B, instead of employing the slitting, pinching and welding technique described with reference to FIGS. 5 and 11 to close the eds of selected fluteways, a star or sunburst pattern metal stamping or casting could be employed. The casting or stamping would simply be welded or otherwise fastened to the end of the fluted tube to close off selected fluteways. In the FIG. 9B embodiment, a single sunburst pattern piece could serve as end plate 55 and to close off the ends of inside fluteways 64.

Cold air inlet 42 extends coaxially within core tube 54 and terminates in a flange 43. Flange 43 is welded to the interior wall of core tube 54 adjacent the beginning of perforated section 65. (FIGS. 9 and 9B).

The fluted sheet 56 shown in FIG. 10 has slits cut therein in a manner different from that shown in the sheet of FIG. 3. Referring to FIG. 10, fluted sheet 56 has a top edge t, a bottom edge b, and left hand and right hand edges indicated by l and r, respectively. A series of parallel fold lines indicated by the numeral 68 show the lines along which sheet 56 will be folded to form the peaks 68 of the fluteways. A series of parallel fold lines indicated by the numeral 70 similarly indicate the fold lines along which troughs 70- will be formed. This much of sheet 56 in FIG. 10 is identical to that of FIG. 6. A difference however, resides in that in the FIG. 10 embodiment all the slit lines 72 are at the edges of fold lines 70 at both the left edge I and the right edge r. No slits are provided at the edges of fold lines 68. After slitting along the slit lines 72 and folding sheet 56 along its fold lines to form it into fluted configuration identical (except for location of the slit lines) to that of FIGS. 4 and 4A, the flute wall material adjacent slit lines 72 are pinched together and welded along weld lines 76 as shown in FIG. 11. The extent of a typical weld lines .is shown by the bracket in FIG. 11. This procedure is carried out on both left hand edge I and right hand edge r of the fluted sheet 56 of FIG. 10. Each inside fluteway 64 is therefore closed at both ends by the weld along line 76. Each outside fluteway 62 is accordingly open at both ends. The completed fluted sheet 56 is wrapped around core tube 54 and edges 1 and b joined along their respective lengths to form a tube, as best seen in FIG. 9B. Slit edges 72 are continuously sealed in a gas-tight manner (e.g., a continuous weld) to core to 54 at both ends of fluted sheet 56. This seals all perforations 80 in sections 65 and 66 beneath the inside surface of fluted sheet 56.

Since the modified form of fluted sheet construction shown in FIG. 11 and utilized in the FIG. 9 embodiment has inside fluteways 64 closed at both ends, two perforated sections 65, 66 of core tube 54 are needed to provide entry into and exit from inside fluteways 64 for the cold air to be heated therein. As indicated above, sheet 56 must completely cover all the perforations 80.

Reinforcing bands 82 are employed to reinforce and insure maintenance of proper spacing between fluteways 62, 64.

Discharge section 48 of exit leg 14 leads to a gas discharge flue (not shown in FIG. 9).

In operation, cold air is admitted via air inlet 42 and travels the length thereof (as shown by double line arrows 53) into perforated section 65 of core tube 54. The useful life of the material of perforated section 65 (including end plate 55) is prolonged by the cooling effect of the cold air thereon, which prevents excessive heating thereof by the hot entry gases. The air passes from perforated section 65 through perforations 80 into inside fluteways 64 as indicated by the double line arrows 53. Hot combustion product gases (indicated by the single line arrows 52) enter outside fluteways 62 from exit leg 14. The hot gases are constrained to flow substantially entirely within fluteways 62 by the jacket tube formed by the interior surface of exit leg 14. The hot gases flow through fluteways 62 concurrently, i.e., in the same direction with and parallel to, the air flow through inside fluteways 64. In the course of this concurrent passage the air is heated by indirect heat exchange with the hot combustion gases through the extended surface of fluted sheet 56. The combustion product gases, having given up a portion of their heat to the air, exit via outside fluteways 62 (as indicated by the arrows 52) into discharge section 48 of exit leg 14 thence to a flue discharge (not shown).

The heated air (as shown by arrows 53) exits from inside fluteways 64 through perforations 80 in perforated section 66, into the annular space formed between core tube 54 and cold air inlet 42. From there, the heated air enters into heated air crossover pipe 44 from whence it is introduced as heated combustion air into entry leg as described hereinabove.

(The use of terms such as gas, air, hot air, etc. to describe pipes and other means in the foregoing preferred embodiments is convenient, but it is not to be taken as implying limitations on the use thereof. The equipment described is obviously utilizable for any two fluids which are to be heat-exchanged.)

The extended surface for indirect heat exchange which is efficiently and economically provided by the heat exchanger of the invention is much more efficient than prior art coaxial conduit recuperators.

The recuperators in accordance with the invention have been found to be particularly useful for radiant tube burners of about 70,000 to 750,000 BTU/hr capacity. Fuel savings obtained by the use of recuperators of the invention range from about 18 to 32% as compared to operation withoutheat recovery. This is a larger energy recovery than was obtained with the use of either smooth cylindrical tube recuperators or conventional finned tube recuperators. The recuperators of the invention do not require jetting or highly turbulent air flow and are operable with the usual furnace blowers which generally provide a pressure head of 10 to 27 inches of water. This is amply sufficient to overcome the total pressure drop through the radiant tube and recuperator of the invention.

The increased preheated air temperature made possible by the recuperators of the invention represent a significant savings in heat energy and fuel usuage.v

It will be noted that in the preferred embodiment of FIG. 2, countercurrent flow of the air and combustion gases is attained with a fluted sheet having individual fluteways open at one end and closed at their other end. In the FIG. 9 embodiment, concurrent flow of the air and gas is attained in a heat exchanger wherein fluteways open at both ends alternate with fluteways closed at both ends. However, the heater of the invention is not necessarily so limited.

For example, FIGS. 12 and 12A show schematically a generalized structure for a heat exchanger in accordance with the invention, wherein countercurrent flow between two fluids is utilized with a fluted sheet wherein fluteways closed at both ends alternate with fluteways open at both ends. (This type of fluted sheet is similar to that shown in FIG. 11 and used in the FIG.

12 9 embodiment.) The embodiment of FIG. 12 is not necessarily limited to use as a recuperator.

A jacket tube 94 (which would be comprised by a portion of exit leg 14 in a recuperator installation) has a first fluid inlet 96 and a first fluid outlet 98.

A second fluid inlet 100, of lesser diameter than jacket tube 94, passes through an end wall 97 thereof coaxially into conduit 94. A second fluid outlet 102, also of lesser diameter than tube 94, passes through the other end wall 99 thereof.

Fluted sheet 56 has inside fluteways 64 and outside fluteways 62 formed therein. Each inside fluteway is closed at both ends and each outside fluteways 62 is open at both ends, in a manner substantially corresponding to the fluted sheet 56 shown in FIG. 11.

The respective ends of inlet 100 and outlet 102 extend into the tube formed by fluted sheet 56 a distance sufficient to close off the scalloped slit ends of the flute so as to seal off inside fluteways 64 from outside fluteways 62.

Reinforcing bands 82 are placed about the outer periphery of fluted sheet 56 by being tack welded to the peaks 68 thereof.

A dummy pipe 104 closed at both ends by end walls 104A is disposed coaxially within the tube formed by fluted sheet 56. Dummy pipe 104 is suitably vented to prevent it from bursting during heating due to thermal expansion. Dummy pipe 104 is generally about the same diameter as the fluid conduits 100 and 102, and may be considered a segmented, blocked-off portion thereof, with openings formed by the open section between the segments.

The second fluid enters second fluid inlet 100 as indicated by the single line arrows and flows around dummy pipe 104 into inside fluteways 64. Dummy pipe 104 serves a core buster function, by forcing the flow of the second fluid therearound and at high velocity through inside fluteways 64. The surface of dummy pipe 104 constrains the second fluid to flow within inside fluteways 64 for substantially the length of the tube formed by fluted sheet 56.

The first fluid enters first fluid inlet 96 as indicated by the double line arrows and flows through the outside fluteways 62, being substantially confined therein by the limited annular space between fluteways 62 and the interior wall of jacket tube 94. The first fluid thus flows countercurrently to the second fluid (whose passage through inside fluteways 64 is indicated by dotted single line arrows). Indirect heat exchange is thus attained between the first and second fluids through the extended surface provided by fluted sheet 56. The first fluid then emerges from first fluid outlet 98 and the second fluid emerges through second fluid outlet 102.

FIG. 13 shows another generalized heat exchanger embodiment 40 whherein parallel (concurrent) flow between two fluids is utilized with a fluted sheet wherein each fluteway is open at one end and closed at its other end. Open and closed ends alternate at both edges of the sheet. This type of fluted sheet is similar to that shown in FIG. 5 and used in the embodiment of FIG. 2.

Portions of the embodiment of FIG. 13 identical or substantially similar to those in FIG. 12 are identically numbered. Thus, a jacket tube 94 has a first fluid inlet 96, a first fluid outlet 98, and a second fluid inlet 100.

A second fluid outlet has an enlarged diameter portion 102A which encircles the outside periphery of fluted sheet 56 at end 56B thereof. Enlarged portion 13 102A extends over the scalloped slit edges at end 56B for a sufficient distance to extend somewhat beyond the open portions of inside fluteways 64 to prevent flow communication between fluteways 62 and 64.

Jacket tube 94 has an enlarged exit chamber 95 formed about the periphery thereof to facilitate the flow of the first fluid from outside fluteways 62 into first fluid outlet 98.

Fluted sheet 56 is formed end 568. Inside fluteways 64 are closed a tube and one end 56A thereof encircles one end of second fluid inlet 100. The other end 56B similarly encircles an end of second fluid outlet 102.

Outside fluteways 62 are open at end 56A, and closed at end, 56B. Inside fluteways 64 are closed at end 56A and open at end 56B, in a manner substantially similar to the fluted sheet shown in FIG. 5.

A vented dummy pipe 104 closed at both ends serves a core buster filnction as in the FIG. 12 embodiment.

The first fluid enters fluid inlet 96 as indicated by the double line arrows and flows through outside fluteways 62, being substantially confined therein by the interior wall of jacket tube 94. At the end of outside fluteways 62, the first fluid is diverted by the closed ends thereof into enlarged exit chamber95, thence into first fluid outlet 98.

The second fluid enters second fluid inlet 100 and is diverted by dummy pipe 104 into inside fluteways 64. The second fluid emerges at end 56B from the open ends of inside fluteways 64 into second fluid outlet 102.

In this manner, the first and second fluids flow concurrently in indirect heat exchange, the heat exchange occurring through the extended thin wall surface of fluted sheet 56.

As described in FIGS. 12 and 13, inlet 100 and outlet 102 together with dummy pipe 104 may be considered to comprise a segmented core tube, the ends of the vented dummy pipe segment being closed off so that it serves a core buster function. It will be apparent that equivalent structure could be provided with a unitary core pipe having perforated sections located at the open section between, respectively, the inlet and outlet pipes and the two ends of dummy pipe 104, with end plates 104A welded in place within the unitary core tube.

As above stated, the fluted sheet tube, the jacket tube and the core tube may have any convenient cross sectional configuration, e.g., circular, oval, polygonal or even a complex cross section of combined geometric shapes. (The term tube is not to be construed as requiring a circular cross section, although such is a preferred embodiment.)

As shown in FIG. 14, a heat exchanger has a fluted sheet tube, a jacket tube and a core tube of rectangular cross section. A rectangular jacket tube 194 enclosed a fluted sheet 156 which is formed into a tube of rectangular cross section around core tube 154, itself of rectangular cross section.

A series of inside fluteways 164 and outside fluteways 162 are formed in fluted sheet 156. The rectangular configuration of this particular embodiment more readily permits an increased depth of individual fluteways as compared to their width. This provides an excellent heat exchange surface area formed by fluted sheet 156. Fluted sheet 156 can be made from a single sheet of material or, if convenience dictates, from two separate sheets joined at their respective edges as indicated at e and e. it will be appreciated that such rectangular (or any other suitable section) fluted heat ex- 14 changer can be provided in any of the embodiments illustrated hereinabove. In short, the embodiments described in this application are not necessarily limited to the circular cross section configurations shown.

As also stated hereinabove, the fluted sheet heat exchangers of the invention are not necessarily limited to use as recuperators in radiant tube heaters. For example, referring now to FIG. 15, there is illustrated schematically a recuperator in accordance with the invention utilized in the flue of a furnace chamber. An enclosed fumace chamber generally indicated by the letter A is enclosed by furnace floor 200, furnace walls 202 and 204, and a furnace roof 206. A suitable burner 208 is positioned in wall 204. Roof 206 has a flue 210 formed therein. Disposed within flue 210 is a recuperator 140 in accordance with the present invention. Recuperator 140 may be of any suitable cross sectional configuration, preferably one which matches the cross section (usually circular or rectangular) of flue 210.

A cold air inlet 142 enters recuperator 140 and cold air is heated therein by indirect heat exchange with hot combustion gases exiting through flue 210, as indicated by the arrows 211. The heat exchanged flue gases exit through an exhaust flue 150. The air heated within recuperator 140 passes through heated air transfer pipe 144 to burner 208 wherein it is admixed with fuel admitted to the burner through fuel supply lines (not shown). The fuel and heated air mixture obtained thereby is combusted in burner 208 and heats furnace enclosure A.

It will be apparent to those skilled in the art that numerous construction details showing well known and conventional items which are not needed for a full explanation of the invention have been omitted from the drawings and description for the sake of simplicity. For example, hot air crossover pipe 44 (FIG. 1) and 144 (FIG. 15) and inlet section 46 (FIG. 1) would normally have heat insulation, not shown in the drawings, provided thereon. Generally, such details as insulation, gaskets, instrumentation, valves, etc. have been omitted where they are not needed for full explanation.

It will also be apparent to those skilled in the art that while the invention has been described in detail with reference to specific embodiments thereof, upon a reading and understanding of the foregoing disclosure many modifications and alterations thereto will occur to those skilled in the art. Such modifications and alterations are nonetheless within the spirit and scope of the described invention. It is intended to include all such modifications and alterations within the scope of the appended claims.

What is claimed is:

1. A heat exchanger comprising a fluted sheet conduit having straight axially extending, radially projecting, inside and outside fluteways formed with peaks and troughs therein, said inside fluteways being defined by the inside surface of said sheet and said outside fluteways being defined by the outside surface of said sheet,

a jacket tube within which said fluted sheet conduit may be slidably inserted,

an imperforate core tube connected in flow communication with said fluted sheet conduit,

means to flow a first fluid through said inside'fluteways between the outside surface of said imperforate core tube"and the inside surface of said fluted sheet conduit, and

means to flow a second fluid through said outside fluteways between the inside surface of said jacket tube and the outside surface of said fluted sheet conduit, whereby there is a heat exchange between said first and second fluids.

2. The heat exchanger of claim 1 wherein said peaks of said fluted sheet conduit are in linear contact with said jacket tube and said troughs of said fluted sheet COldLlit are in linear contact with said imperforate core tu e.

3. The heat exchanger of claim 2 wherein said jacket tube and said core tube are circular in cross section.

4. The heat exchanger of claim 1 wherein said fluted sheet conduit is polygonal in cross section.

5. The heat exchanger of claim 4 wherein said fluted sheet conduit and said core tube are rectangular in cross section.

6. The heat exchanger of claim 1 wherein each fluteway is open at one end and closed at its other end.

7. The heat exchanger of claim 6 wherein: said means to flow said first fluid includes a fluid inlet connected in flow communication to an entry chamber, said entry chamber being connected in flow communicationto the open ends of each inside fluteway; said core tube includes an opening therein which is in flow communication with the closed ends of said inside fluteways; a fluid transfer pipe is in flow communication with said opening and with a fluid outlet conduit through which said first fluid is removed from said heat exchanger.

8. The heat exchanger of claim 7 wherein said opening comprises a perforated section in said core tube.

9. The heat exchanger of claim 7 wherein said jacket tube comprises a. portion of the exit leg of a radiant tube heater and said fluid outlet conduit is a hot air crossover pipe.

10. The heat exchanger of claim 9 wherein said jacket tube, said core tube and said fluted sheet conduit are all circular in cross section, the latter comprising a fluted tubular sheet.

11. The heat exchanger of claim 1 wherein fluteways closed at both ends are alternated with fluteways open at both ends.

12. The heat exchanger of claim 11 wherein said inside fluteways are closed at both ends and said outside fluteways are open at both ends.

13. The heat exchanger of claim 12 wherein said means to flow a first fluid include a first fluid inlet pipe extending through said core tube and into fluid flow communication with a first opening therein, said first opening being in fluid flow communication with said inside fluteways at one end thereof, the other end of said inside fluteways being in fluid flow communication with a second opening in said core tube, and said second opening being in turn connected in flow communication with a fluid outlet conduit.

14. The heat exchanger of claim 13 wherein said first and second openings comprise, respectively, a first perforated section and a second perforated section.

15. The heat exchanger of claim 13 wherein said first and second openings comprise, respectively, a first open section and a second open section.

16. The heat exchanger of claim 13, wherein said jacket tube comprises a portion of the exit leg of a radiant tube heater and said fluid outlet conduit is a hot air crossover pipe.

17. The heat exchanger of claim 16 wherein said jacket tube, said core tube and said fluted sheet conduit are all circular in cross section, the latter comprising a fluted tubular sheet.

18. A heat exchanger comprises a jacket tube having a first fluid inlet and a first fluid outlet connected in fluid flow communication therewith,

a fluted sheet tube disposed within said outer jacket to define a first fluid flow space, said fluted sheet tube having outside fluteways and inside fluteways disposed longitudinally there along, segmented core tube having an inlet portion connected in fluid flow communication to one end of said fluted sheet tube, an outlet portion connected in fluid flow communication to the other end of said tube, and a dummy portion contained within said fluted sheet tube, said core tube inlet portion being adapted to serve as a second fluid inlet and said core tube outlet portion being adapted to serve as a second fluid outlet.

19. The heat exchanger of claim 18 wherein said outside fluteways are open at both ends and said inside fluteways are closed at both ends, said first fluid inlet is in fluid flow communication with one end of said outside fluteways, said first fluid outlet is in fluid flow communication with the other end of said outside fluteways, said inlet portion is connected in fluid flow communication with one end of said inside fluteways and said outlet portion is connected in fluid flow communication with the other end of said inside fluteways.

20. The heat exchanger of claim 18 wherein each of said fluteways is opened at one end and closed at its other end, said first fluid inlet is in fluid flow communication with the open end of said outside fluteways, said first fluid outlet is in fluid flow communication with the closed ends of said outside fluteways, said core tube inlet portion is in fluid flow communication with the closed ends of said inside fluteways and said core tube outlet portion is in fluid flow communication with the open end of said inside fluteways.

21. The heat exchanger of claim 18 wherein said fluted sheet tube is circular in cross section.

22. The heat exchanger of claim 18 wherein said fluted sheet tube is rectangular in cross section.

23. A radiant tube heater comprises a furnace enclosure containing a radiant heating tube therein,

said radiant tube having an entry leg leading into said enclosure, an intermediate radiating section within said enclosure, and an exit leg from said enclosure, said entry leg, radiating section and exit leg being interconnected in sequential fluid flow communication,

the improvement comprising a recuperator positioned within said exit leg, said recuperator including a fluted sheet tube having longitudinally extending inside and outside fluteways formed thereon,

said fluted sheet tube being disposed within said exit leg whereby a segment thereof serves as a jacket tube for said fluted sheet tube,

a cold air inlet connected in fluid flow communication to one end of said inside fluteways, and a hot air transfer pipe connected in fluid flow communication to the other end of said inside fluteways,

a hot air crossover pipe connecting said hot air transfer pipe to said entry leg in fluid flow communication,

means to admit a fuel to said entry leg admixture with hot air admitted therein by said cross-over pipe,

means to ignite said admixed air and fuel within said radiant tube, and 

1. A heat exchanger comprising a fluted sheet conduit having straight axially extending, radially projecting, inside and outside fluteways formed with peaks and troughs therein, said inside fluteways being defined by the inside surface of said sheet and said outside fluteways being defined by the outside surface of said sheet, a jacket tube within which said fluted sheet conduit may be slidably inserted, an imperforate core tube connected in flow communication with said fluted sheet conduit, means to flow a first fluid through said inside fluteways between the outside surface of said imperforate core tube and the inside surface of said fluted sheet conduit, and means to flow a second fluid through said outside fluteways between the inside surface of said jacket tube and the outside surface of said fluted sheet conduit, whereby there is a heat exchange between said first and second fluids.
 2. The heat exchanger of claim 1 wherein said peaks of said fluted sheet conduit are in linear contact with said jacket tube and said troughs of said fluted sheet conduit are in linear contact with said imperforate core tube.
 3. The heat exchanger of claim 2 wherein said jacket tube and said cOre tube are circular in cross section.
 4. The heat exchanger of claim 1 wherein said fluted sheet conduit is polygonal in cross section.
 5. The heat exchanger of claim 4 wherein said fluted sheet conduit and said core tube are rectangular in cross section.
 6. The heat exchanger of claim 1 wherein each fluteway is open at one end and closed at its other end.
 7. The heat exchanger of claim 6 wherein: said means to flow said first fluid includes a fluid inlet connected in flow communication to an entry chamber, said entry chamber being connected in flow communication to the open ends of each inside fluteway; said core tube includes an opening therein which is in flow communication with the closed ends of said inside fluteways; a fluid transfer pipe is in flow communication with said opening and with a fluid outlet conduit through which said first fluid is removed from said heat exchanger.
 8. The heat exchanger of claim 7 wherein said opening comprises a perforated section in said core tube.
 9. The heat exchanger of claim 7 wherein said jacket tube comprises a portion of the exit leg of a radiant tube heater and said fluid outlet conduit is a hot air crossover pipe.
 10. The heat exchanger of claim 9 wherein said jacket tube, said core tube and said fluted sheet conduit are all circular in cross section, the latter comprising a fluted tubular sheet.
 11. The heat exchanger of claim 1 wherein fluteways closed at both ends are alternated with fluteways open at both ends.
 12. The heat exchanger of claim 11 wherein said inside fluteways are closed at both ends and said outside fluteways are open at both ends.
 13. The heat exchanger of claim 12 wherein said means to flow a first fluid include a first fluid inlet pipe extending through said core tube and into fluid flow communication with a first opening therein, said first opening being in fluid flow communication with said inside fluteways at one end thereof, the other end of said inside fluteways being in fluid flow communication with a second opening in said core tube, and said second opening being in turn connected in flow communication with a fluid outlet conduit.
 14. The heat exchanger of claim 13 wherein said first and second openings comprise, respectively, a first perforated section and a second perforated section.
 15. The heat exchanger of claim 13 wherein said first and second openings comprise, respectively, a first open section and a second open section.
 16. The heat exchanger of claim 13, wherein said jacket tube comprises a portion of the exit leg of a radiant tube heater and said fluid outlet conduit is a hot air crossover pipe.
 17. The heat exchanger of claim 16 wherein said jacket tube, said core tube and said fluted sheet conduit are all circular in cross section, the latter comprising a fluted tubular sheet.
 18. A heat exchanger comprises a jacket tube having a first fluid inlet and a first fluid outlet connected in fluid flow communication therewith, a fluted sheet tube disposed within said outer jacket to define a first fluid flow space, said fluted sheet tube having outside fluteways and inside fluteways disposed longitudinally there along, a segmented core tube having an inlet portion connected in fluid flow communication to one end of said fluted sheet tube, an outlet portion connected in fluid flow communication to the other end of said tube, and a dummy portion contained within said fluted sheet tube, said core tube inlet portion being adapted to serve as a second fluid inlet and said core tube outlet portion being adapted to serve as a second fluid outlet.
 19. The heat exchanger of claim 18 wherein said outside fluteways are open at both ends and said inside fluteways are closed at both ends, said first fluid inlet is in fluid flow communication with one end of said outside fluteways, said first fluid outlet is in fluid flow communication with the other end of said outside fluteways, said inlet portion is connected in fluid flow communication With one end of said inside fluteways and said outlet portion is connected in fluid flow communication with the other end of said inside fluteways.
 20. The heat exchanger of claim 18 wherein each of said fluteways is opened at one end and closed at its other end, said first fluid inlet is in fluid flow communication with the open end of said outside fluteways, said first fluid outlet is in fluid flow communication with the closed ends of said outside fluteways, said core tube inlet portion is in fluid flow communication with the closed ends of said inside fluteways and said core tube outlet portion is in fluid flow communication with the open end of said inside fluteways.
 21. The heat exchanger of claim 18 wherein said fluted sheet tube is circular in cross section.
 22. The heat exchanger of claim 18 wherein said fluted sheet tube is rectangular in cross section.
 23. A radiant tube heater comprises a furnace enclosure containing a radiant heating tube therein, said radiant tube having an entry leg leading into said enclosure, an intermediate radiating section within said enclosure, and an exit leg from said enclosure, said entry leg, radiating section and exit leg being interconnected in sequential fluid flow communication, the improvement comprising a recuperator positioned within said exit leg, said recuperator including a fluted sheet tube having longitudinally extending inside and outside fluteways formed thereon, said fluted sheet tube being disposed within said exit leg whereby a segment thereof serves as a jacket tube for said fluted sheet tube, a cold air inlet connected in fluid flow communication to one end of said inside fluteways, and a hot air transfer pipe connected in fluid flow communication to the other end of said inside fluteways, a hot air crossover pipe connecting said hot air transfer pipe to said entry leg in fluid flow communication, means to admit a fuel to said entry leg admixture with hot air admitted therein by said cross-over pipe, means to ignite said admixed air and fuel within said radiant tube, and means to pass the products of combustion obtained thereby in fluid flow communication through said outside fluteways.
 24. The radiant tube heater of claimm 23 including a cold air entry chamber connecting said cold air inlet in fluid flow communication to said one end of said inside fluteways.
 25. The radiant tube heater of claim 23 wherein a core tube extends within said fluted sheet tube generally coaxially therewith, said hot air transfer pipe extends generally coaxially within said core tube and said core tube contains an opening therein which connects said other end of said inside fluteways in fluid flow communication with said hot air transfer pipe.
 26. The radiant tube heater of claim 25 wherein said opening comprises a perforated section of said core tube.
 27. The heat exchanger of claim 23 further including a core tube which extends generally coaxially through said fluted sheet tube, and which has first and second openings formed therein, with said cold air inlet extending generally coaxially through said core tube, said cold air inlet being in fluid flow communication with said first opening, said first opening being in fluid flow communication with one end of said inside fluteways, and said second opening being in fluid flow communication with one end of said inside fluteways and thence with said hot air crossover pipe, and said inside fluteways are closed at both ends and said outside fluteways are open at both ends.
 28. The heat exchanger of claim 27 wherein said first and second openings comprise, respectively, first and second perforated sections in said core tube.
 29. A heat exchanger adapted to be slidably received within a jacket tube comprising: an imperforate core tube; a fluted heat exchanger tube surrounding said core tube and having straight exterior and interior fluteways with peaks and troughs extending along the longitudinal axIs of said core tube, said peaks bein in close proximity with the interior wall of said jacket tube and said troughs being in close proximity with said core tube; an imperforate fluid transfer tube spaced coaxially within said core tube; means to flow a first fluid in the spaced defined by the core tube and said interior fluteways; means to flow a second fluid in the spaces defined by the jacket tube and said exterior fluteways; means to connect said spaces defined by the core tube and the interior fluteways with the interior of said fluid transfer tube; second fluid exhaust means; and means to connect said spaces defined by the jacket tube and said exterior fluteways with said second fluid exhaust means.
 30. The heat exchanger of claim 29 wherein said means to connect said spaces defined by the core tube and the interior fluteways with the interior of said fluid transfer tube comprises a perforated extension of said core tube.
 31. The heat exchanger of claim 29, wherein the interior fluteways of said heat exchanger tube are open at a first fluid receiving end of said heat exchanger tube and closed at the opposite end of said heat exchanger tube; and the exterior fluteways of said heat exchanger tube are open at said opposite end of said heat exchanger tube and closed at said first fluid receiving end, wherein said first fluid flows through said interior fluteways and said second fluid flows through said exterior fluteways, communication between said first and second fluids being prevented by the arrangement of said open and closed fluteway ends.
 32. The heat exchanger of claim 29, including means to direct flow of said first and second fluids in opposite directions in their respective fluteways.
 33. The heat exchanger of claim 29, wherein said interior fluteways are closed at opposite ends and the exterior fluteways are open at opposite ends, whereby said first fluid flows through said interior fluteways and said second fluid flows through said exterior fluteways, and means to prevent communication between said first and second fluids.
 34. The heat exchanger of claim 29, wherein interior fluteways are open at opposite ends and the exterior fluteways are closed at opposite ends, whereby said first fluid flows through said interior fluteways and said second fluid flows through said exterior fluteways, and means to prevent communication between said first and second fluids.
 35. A heat exchanger adapted to be slidably received within a jacket tube comprising: an imperforate core tube; a fluted heat exchanger tube surrounding said core tube and having straight exterior and interior fluteways with peaks and troughs extending along the longitudinal axis of said core tube, said peaks being in close proximity with the interior wall of said jacket tube and said troughs being in close proximity with said core tube; an imperforate fluid transfer tube spaced coaxially within said core tube; communicating means between said transfer tube and said interior fluteways; communicating means between said interior fluteways and the interior of said core tube; first fluid exhaust means in said core tube; second fluid exhaust means on the exit end of said jacket tube means to flow a first fluid through said transfer tube, said interior fluteways, the interior of said core tube and said first fluid exhaust means; and means to flow a second fluid through said exterior fluteways and said second fluid exhaust means, whereby there is a heat exchange between said first and second fluids.
 36. A heat exchanger adapted to be slidably received within a jacket tube comprising: an imperforate core tube; a fluted heat exchanger tube surrounding said core tube and receivable within said jacket tube; said fluted heat exchanger tube having straight exterior and interior fluteways with peaks and troughs extending along the longitudinal axis of said core tube, said peaks and troughs being in close proximiTy to the interior of said jacket tube and the exterior of said core tube respectively; fluid ingress and egress means on opposite ends of said jacket tube; fluid ingress and egress means at opposite ends of said fluted heat exchanger in communication with the interior of said core tube; means to flow a first fluid through said core tube, said ingress means, said interior fluteway and said egress means; means to flow a second fluid through said jacket tube ingress means; said exterior fluteway and said jacket tube egress means; whereby there is a heat exchange between said first and second fluids.
 37. The heat exchanger of claim 36, wherein said first and second fluids are adapted to counterflow relative to each other.
 38. The heat exchanger of claim 36, wherein said first and second fluids are adapted to parallel flow relative to each other. 